Allosteric regulation of α_IIbβ₃ by β₃ 95–105

Summary

The linear recognition sequences of an anti-β₃ antibody that blocked platelet aggregation were identified using β₃ tryptic peptides. Two of these recognition sequence-containing peptides were mapped to β₃ 92–105, and antibodies affinity purified using these peptides blocked platelet aggregation. Examining the structure of α_IIbβ₃ identified β₃ 95–105 as the solvent accessible sequence within β₃ 92–105. A peptide corresponding to β₃ 95–105 was synthesized and used to affinity purify the β₃ antibody. Anti-β₃ 95–105 completely blocked platelet aggregation and agonist-induced fibrinogen binding to platelets, but had no effect on cyclic-RGD binding. Binding of anti-β₃ 95–105 to α_IIbβ₃ also did not alter the structure of the α_IIb cap subdomain, as measured by anti-α_IIb 201–217 binding. β₃ 95–105 and peptides spanning two adjacent sequences in the structure of β₃ did not bind fibrinogen and were ineffectual in blocking agonist-induced platelet aggregation. Structure analysis revealed that β₃ 95–105 is adjacent to one of the two hinges in β₃ that allows for the outward swing of the hybrid and PSI domains which is central to the conversion of α_IIbβ₃ from a low into a high affinity state.Thus, the binding of an antibody to β₃ 95–105 could serve as a fulcrum for allosteric regulation of α_IIbβ₃ by regulating the movement of the hybrid-PSI domain.

Keywords

Antiaggregatory agents - fibrinogen binding - GPIIb/IIIa inhibitors - platelet activation inhibitors

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